Previously known are, for instance from patent specifications FI 933074 and GB 2 255 365, double bit assemblies where the inner bit that drills the center hole can be pulled out from the hole, while the ring bit that drills the outer hole circle is left in the bore. However, in these examples the inner bit can be pulled out from the hole after drilling and also reinstalled if drilling is continued.
On drilling a hole in the soil, the most general case is that the first portion of the hole is made in soft soil, whereby a protection tube is pulled into the drilled hole. Eventually, on hitting rock, the protection tube is not needed anymore. Drilling can then be continued without a protection tube if change of the drilling procedure is easy. A problem arises when the drilling procedure is changed. The inner bit must be pulled out from the hole and replaced by a bit that drills a hole with a smaller diameter. During this replacement, the complete drilling equipment is pulled out from the hole and reinstalled furnished with a new rock-drilling bit. This takes time and money, especially if the first bore in the soil is long.